Oligomerization of ethylene to liquid transportation fuels with post synthesis treated zsm-5 catalyst

ABSTRACT

A process for post synthesis treatment of ZSM-5 catalyst for converting ethylene to liquid fuel products providing substantially improved catalyst life. The treatment comprises either a base treatment, an acid treatment or a two-step treatment where one is with an acid and the other is with a base. The base treatment is provided by a weak sodium hydroxide such as less than 1 Molar concentration. The acid treatment is stronger acid where, for example, a hydrogen chloride solution at greater than 2 Molar concentration is used.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

This invention relates to converting light alkanes to fuel andespecially to improved catalysts for the economic and practicalimplementation of a commercial conversion process.

BACKGROUND OF THE INVENTION

The US shale gas boom has resulted in a significant increase in naturalgas production as well as a significant increase in the production ofnatural gas liquids. One of the main components of the natural gasliquids produced with natural gas is ethane. Ethane is most commonlyused as petrochemical feedstock such as for the production of ethylene.Ethylene is a feedstock for many, many high volume chemical basedproducts such as polyethylene and styrene plastics, among many others.However, there are no other sizable consumption markets for ethane. USethane supplies currently exceed demand by about 300,000 barrels per daycausing depressed prices for ethane and attracting considerableinvestment into new ethane to ethylene production facilities. Mostsupply/demand estimates indicate that ethane will remain in surplus formany years and these predictions take in to account the new ethane toethylene conversion capacity being built. Therefore, new markets forethane and new technologies for converting ethane to products that havelarge existing or substantially growing demand would be very attractivein light of the projected low prices for ethane for many years. One ofthe largest end use markets is liquid transportation fuel and a simpleconversion technology to any transportation fuel could prove to be quiteprofitable.

So, with the expectation that ethane will be plentiful and cheap, oldtechnologies are being reconsidered that use ethane as a feedstock. Oneold technology is the conversion of ethane to current fuel markets suchas gasoline and/or diesel. However, while there are known chemicalprocesses for converting ethane to gasoline and or diesel, it has yet tobe put into commercial production. With excess ethane currently beingproduced, there is or will soon be a need to create commercially viableprocesses to convert ethane to liquid transportation fuels.

BRIEF SUMMARY OF THE DISCLOSURE

The invention more particularly relates to oligomerizing ethylene totransportation fuel products in a reactor with a fixed bed of ZSM 5catalyst that is essentially free of catalyst metals other than silicaand alumina. The ZSM 5 catalyst is provided with post synthesis acidtreatment to resist coke formation in the zeolite crystallites andextend catalyst life. The oligomerizing is conducted at a pressurebetween 0 psig and 800 psig, a temperature of between 260° C. and 420°C., and a gas hourly space velocity of between 1000 and 5000 inversehours wherein at least 85% of the ethylene is converted.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefitsthereof may be acquired by referring to the follow description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic drawing of a reactor for the present invention;

FIG. 2 is a chart showing catalyst life for ethylene conversion in theschematic process according to FIG. 1 for various catalysts; and

FIG. 3 is a chart showing improved catalyst life over and above thatshown in FIG. 2 obtained by the catalyst treatment described in thepresent invention.

DETAILED DESCRIPTION

Turning now to the detailed description of the preferred arrangement orarrangements of the present invention, it should be understood that theinventive features and concepts may be manifested in other arrangementsand that the scope of the invention is not limited to the embodimentsdescribed or illustrated. The scope of the invention is intended only tobe limited by the scope of the claims that follow.

Ethane may be converted to liquid transportation fuel in a process 10shown schematically in FIG. 1. Ethane from an ethane stream 12 is feddirectly into a cracking furnace 14. The process of cracking ethane istypically an uncatalyzed process relying on pressure and temperature ina furnace, such as cracking furnace 14. However, there are catalyticprocesses for cracking ethane that are acceptable for the presentinvention. The cracking furnace 14 produces a number products primarilyincluding hydrogen, ethylene, water, methane, and unconverted ethane,but also including small amounts of propylene, acetylene and butadiene,along with trace amounts of other hydrocarbons. All of these productsare suitable for conversion in an ethylene oligomerizer in accordancewith the present invention.

The products from the cracking furnace 14 are fed directly to a quenchtower 16 via a furnace conduit 15 to stop further thermal reactions. Aliquid product is taken from the bottom of the quench tower 16 via drain18 comprising gasoline and fuel oil density materials that may be fed toa refinery. The bulk of the products are vaporous under the conditionsin the quench tower 16 and exit the top via overhead conduit 17. Thebulk of the products comprise raw ethylene along with the other lighterproducts described above.

The raw ethylene stream is fed into a catalytic oligomerization reactor20. The catalytic oligomerization reactor 20 includes a fixed bed ofcatalyst to convert the raw ethylene stream into a number of productsprimarily including a gasoline product having an octane rating of about88. The products from the catalytic oligomerization reactor 20 areconveyed via an outlet conduit to a product separator 22 to separate theproducts into at least three streams or cuts. A bottom cut comprising astream of C4 to C15 hydrocarbon molecules that exits the bottom viaproduct conduit 23. These are the valuable products that may be blendedinto current liquid transportation fuels such as gasoline and perhapsdiesel or jet fuel.

The top cut comprises light gases such as hydrogen and C1 to C3hydrocarbons that are conveyed back to the furnace 14 via light gasconduit 24. These light gases are burned in the furnace 14 to generateheat or supplemental heat for cracking the ethane. A middle cutcomprising light hydrocarbons having a C3 to C5 chain length exitsmiddle cut conduit 26. The light hydrocarbons may be recycled to thecatalytic oligomerization reactor 20 via recycle conduit 27 or may bepurged from the system 10 via purge line 28. If the middle cut is to bepurged, it is preferably directed into a refinery stream or to a NGLfractionator for capture and sale.

This process is fairly simple with one well known and well understoodunit (the furnace 14), and a second unit (the oligomerization reactor20) that been considered over the years. Interestingly, in currentstudies, although there are other products in the feed stream to thecatalytic oligomerization reactor 20, in early tests, about 98% of theethylene was converted and over 75% of the products from the ethyleneconverted to C5+ materials (Table 1). This is very exciting. With ethaneprices depressed and gasoline prices being among the highest pricednon-specialty refinery products, this conversion technology could proveto be quite profitable.

The operating conditions may be in a range where the oligomerizing isconducted at a pressure of between 0 psig and 800 psig, the temperatureis maintained in a range of between 260° C. and 420° C., and the feedrate measured as a gas hourly space velocity is in a range of between1000 and 5000 inverse hours. While higher productivity is desired,ideally at least 85% of the ethylene is converted.

So, while this kind of system is exciting, there is a down side. Thedown side is that although there are quite a number of known catalystsfor the catalytic process of oligomerizing ethylene, the catalyst lifeof these catalysts is desperately short for a financially viablecommercial system. Many catalysts have been tested. The best results sofar have been accomplished with ZSM-5 catalysts, but the catalyst lifeof ZSM-5 catalysts still has been measured in hours and has been farshorter than economically viable in a commercial application. Whilethere are many ZSM-5 catalysts produced by many vendors and many havebeen tested, the productivity and catalyst life tends to vary quitesubstantially. The basic formulation and sieve structure is the same,but the range of crystallite sizes tend to vary along with the range ofcatalyst particle sizes, along with the sizes of the micropores andmesopores. There may be many other differences from one manufacturer toanother even though the sieve size is quite standardized as set by thecrystal structure of alumina and silica and there are no other catalystmetals or materials otherwise added to the ZSM-5 catalyst.

Basically, it is believed that the catalysts tend to coke up pretty fastand while the catalysts may be regenerated through conventionaloxidation by burning the coke off the catalyst, such short catalyst lifebetween regeneration cycles will create substantial operating costs.While the current price spread between ethane and gasoline is large andvery attractive, price spreads will change over time, most likelyshrinking and not expanding. With high operating costs, even a marginalreduction in price spread could substantially reduce or eliminate anyprofitability of such a commercial system.

TABLE 1 Pressure, psig 50 Temperature, Degrees Celsius 310 Ethyleneconversion, % 98 HC product selectivity, wt % Methane 0.1 Ethane 0.8Propane 2.1 Propylene 1.9 Butanes 9.3 Butenes 5.5 C5+ 80.2 Total, %100.00

Focusing on catalyst life, the invention may be illustrated by showingwhere commercial catalysts A and B were tested in a bench scale reactorat 50 psig and 310° C. High conversion of the ethylene was the focus andas shown in FIG. 2, Catalyst A lost substantial conversion productivityright away while Catalyst B maintained high conversion. However, highconversion for only 100 hours is probably quite a bit short of beingsuitable for a prospective commercial operation.

Focusing on avoiding coke formation to increase catalyst life, it isbelieved that the process of forming coke in this reaction system isactually a multistep reaction process that begins with oligomerization,then proceeds to cyclization, then to poly-nuclear cyclization and thenfinally to coke.

This present invention is focused on doing post synthesis treatments forZSM-5 catalysts to make them more resistant to coke formation whileconverting ethylene to liquid hydrocarbon products. So, the ZSM-5 hasalready been produced by the manufacturer and the inventive processrelates to altering the catalyst post production to minimize theproduction of coke.

The inventive process comprises a one or two step process for the ZSM-5catalyst. One process is an acid treatment. As Catalyst B appears toperform better than Catalyst A, the post production efforts are focusedon Catalyst B. A sample of Catalyst B is subjected to a 4 Molarconcentration HCl (hydrogen chloride) solution for 2 hours at 80° C. Thesample is then thoroughly washed again in distilled water and dried at120° C. for 10 hours. Finally, the sample of Catalyst B (labeled AcidTreated Catalyst B) is calcined at 550° C. for 6 hours. The Acid TreatedCatalyst B was tested in a lab scale ethylene conversion process shownin FIG. 1 and the conversion and time on stream are shown in FIG. 3.

Another process is a base treatment. A sample of Catalyst B is exposedto a 0.01 Molar concentration of NaOH (sodium hydroxide) at 40° C. for0.5 hours. The temperature is raised after the initial 30 minutes to 60°C. to continue for an additional 60 minutes at the same NaOHconcentration. The sample is washed in distilled water and dried at 120°C. for 10 hours. Finally, the sample of Catalyst B (labeled Base TreatedCatalyst B) is calcined at 550° C. for 6 hours. Base Treated Catalyst Bwas tested in lab scale ethylene conversion process shown in FIG. 1 andthe conversion and time on stream are shown in FIG. 3.

The last process is a two-step process comprising both an acid treatmentand a base treatment. A sample of Catalyst B is exposed to a 0.01 Molarconcentration of NaOH (sodium hydroxide) at 40° C. for 0.5 hours. Thetemperature is raised after the initial 30 minutes to 60° C. to continuefor an additional 60 minutes at the same NaOH concentration. The sampleis washed in distilled water and then subject to an acid wash for 2hours at 80° C. in a 4 Molar concentration HCl (hydrogen chloride)solution. The sample of Catalyst B (labeled Base-Acid Treated CatalystB) is thoroughly washed again in distilled water and dried at 120° C.for 10 hours and then calcined at 550° C. for 6 hours. The two stepprocess may be undertaken in either order, but the better results wereobtained with the Base treatment being taken first with the Acidtreatment second. The Base-Acid Treated Catalyst B was tested in a labscale ethylene conversion process shown in FIG. 1 and the conversion andtime on stream are shown in FIG. 3.

One of the concerns for such treatments was harm to the underlyingframework Si/A1 ratio. To assess whether the catalyst was being alteredin a manner that would reduce the number of regeneration cycles thecatalyst could endure, the framework Si/A1 ratios were determined bysolid-state NMR for each sample. Referring to Table 2 below, the Si/A1ratios are reported for Catalyst B without post treatment, Acid TreatedCatalyst B, Base Treated Catalyst B and the Base-Acid Treated CatalystB. These results suggest that none of the acid treatment, the basetreatment or the base-acid treatments have much impact on ZSM-5framework Si/A1 ratios. X-ray diffraction patterns also indicate thatthe treatments did not destroy the ZSM-5 structure in Catalyst B. Bothof these results are unexpected findings. This is probably due to thepresence of alumina binder in Catalyst B.

TABLE 2 Catalyst Framework Si/Al ratio by NMR Catalyst B (withouttreatment) 35 Acid treated Catalyst B 36 Base treated Catalyst B 39Base-Acid treated Catalyst B 38

Referring to FIG. 3, it should be appreciated that each of the proposedtreatments provide improved catalyst life measurements over Catalyst Bwithout either or both the acid treatment and base treatment in theprocess described in FIG. 1 and under the same conditions which producedthe previously measured catalyst life data shown in FIG. 2. Thecomparison of the sample catalyst produced conversion productivity andcatalyst life data shown in FIG. 3 provides a profound improvement incatalyst life.

The tests were performed under the conditions shown in Table 1 with arepresentative product slate and performance. These conditions areadequate to compare the stability of the catalysts. For a viablecommercial swing fixed bed operation, it is desirable to have about 10days of catalyst life before it becomes necessary to regenerate thecatalyst and at least about a year before replacement of the catalystbecomes necessary, assuming that the catalyst will have undergone manycycles of online production and offline regeneration. The base-acidtreated Catalyst B was approaching 10 days online before its ethyleneconversion dropped below 90%. This suggests that the minimum targetcatalyst life for commercial operation is getting close and theinventive process provides a significant step towards that goal.Operational parameters and other developments may further improvecatalyst life making the potential commercial operation moreeconomically viable.

It should be recognized that other acids and other base solutions may beused other than HCL and NaOH. And the conditions are not intentionallylimited to what is described in the above examples. For example, thebase treatment of sodium hydroxide may be in a range of between about0.001 and about 0.5 Molar concentration although it is anticipated thatit might better be in a range of between about 0.005 and about 0.25Molar concentration or between about 0.0075 and about 0.15 Molarconcentration. Similarly, the acid treatment of hydrogen chloride may bein a range of between about 1 and 10 Molar concentration, but mightbetter be in a range of between about 2 and 7 Molar concentration orbetween about 3 and 5 Molar concentration. As noted, it is preferredthat the catalyst be dried and calcined after the acid, base or two steptreatment is completed. Drying is preferred at a temperature above 105°C. and calcining is preferred at a temperature above 400° C. The actualtreatments may also be preferred at elevated temperatures of betweenabout 35° C. and 90° C.

In closing, it should be noted that the discussion of any reference isnot an admission that it is prior art to the present invention,especially any reference that may have a publication date after thepriority date of this application. At the same time, each and everyclaim below is hereby incorporated into this detailed description orspecification as additional embodiments of the present invention.

Although the systems and processes described herein have been describedin detail, it should be understood that various changes, substitutions,and alterations can be made without departing from the spirit and scopeof the invention as defined by the following claims. Those skilled inthe art may be able to study the preferred embodiments and identifyother ways to practice the invention that are not exactly as describedherein. It is the intent of the inventors that variations andequivalents of the invention are within the scope of the claims whilethe description, abstract and drawings are not to be used to limit thescope of the invention. The invention is specifically intended to be asbroad as the claims below and their equivalents.

1. Oligomerizing ethylene to transportation fuel products in a reactorwith a fixed bed of ZSM-5 catalyst that is essentially free of catalystmetals other than silica and alumina wherein the ZSM-5 catalyst has beenprovided with post synthesis acid treatment to resist coke formation inthe zeolite crystallites and extend catalyst life wherein theoligomerizing is conducted at a pressure between 0 psig and 800 psig, atemperature of between 260° C. and 420° C., and a gas hourly spacevelocity of between 1000 and 5000 inverse hours wherein at least 85% ofthe ethylene is converted.
 2. The process according to claim 1 whereinthe post synthesis treatment of the catalyst comprises catalyst an acidtreatment with hydrogen chloride.
 3. The process according to claim 2wherein the acid treatment is a hydrogen chloride solution at betweenabout 1 and 10 Molar concentration.
 4. The process according to claim 3wherein the acid treatment is between about 2 and 7 Molar concentration.5. The process according to claim 4 wherein the acid treatment isbetween about 3 and 5 Molar concentration.
 6. The process according toclaim 2 wherein the post synthesis treatment of the catalyst furthercomprises the steps of drying catalyst and calcining the catalyst afterthe acid treatment.
 7. The process according to claim 6 wherein the postsynthesis treatment of the catalyst further comprises the steps ofdrying catalyst at a temperature above 105° C. and calcining thecatalyst after the acid treatment at a temperature above 400° C.
 8. Theprocess according to claim 1 wherein acid treatment of the catalyst isdone at elevated temperature between 35° C. and 90° C.